This invention relates to a scanning electron microscope (SEM) which can observe minute objects on the surface of a specimen.
A scanning electron microscope (SEM) scans a specimen with electron beams, detects secondary electrons emitted from the specimen as the result of bombardment by electron beams, and displays a secondary electron image representing the scanned objects on a display screen. This technology is also used for observation of minute structures in semiconductor manufacturing fields. Recently, as semiconductor devices have quickly become smaller and smaller, objects and defects on specimens have also become much smaller. They are too small to be searched and detected by conventional optical object/defect investigating devices and the like. Their resolutions have almost reached the limits of searches and observations of objects and defects. In production of such minute semiconductor devices, foreign objects and defects of micro sizes on silicone wafers may cause errors and problems. Further each silicone wafer has a lot of objects and defects to be observed (e.g. some tens to some hundreds. For detailed investigation of these objects and defects, the semiconductor manufacturers have been longing for means for automatic observation and investigation which combine an Auto Defect Review (ADR) function by a scanning electron microscope (SEM) and an Auto Defect Classification (ADC) function which automatically classifies objects and defects detected by the ADR function.
Usually to observe such minute objects and defects, the SEM takes the steps of locating objects and defects on each wafer in advance by another inspection system, searching and observing them according to the coordinate data of their positions. However, substantially, there is a slight difference between the coordinates system of the inspection system and the coordinates system of the SEM and this difference (error) is one of the factors which prevent automation of investigation of objects and defects by the SEM. Usually, the SEM magnifies the secondary electron image of objects and defects of submicron sizes by at least 5000 times to display it on the screen of the SEM. Naturally, the size of the SEM screen is limited and the area you can investigate at a time is also limited. Therefore, if the positional data of an object or defect obtained by the inspection system contains a large error, the image of the object or defect may not be In the SEM screen. For example, you can observe an area of only 40 xcexcm square at a time on the SEM screen of 200 mm square at a magnification of xc3x975000. If the object/defect coordinate data obtained by the inspection system contains an error of xc2x120 xcexcm or more, the image of the object/defect is not in the screen area and you cannot find it.
One of methods to solve the above problem comprises the steps of obtaining coordinate data of objects or defects of known sizes on a test specimen by an inspection system, obtaining the coordinate data of the objects or defects on the test specimen in the SEM coordinates system, determining a coordinate converting expression to minimize their coordinate difference, and using this expression for fine positional adjustment. However, if the correction is made by objects or defects which are selected at random, the correction values may be various and the result of correction will not always be assured. To solve such as problem, Japanese Non-examined Patent Publications H11-167893 (1999) (titled xe2x80x9cScanning electron microscopexe2x80x9d) discloses a method of automatically re-calculating a coordinate converting expression using only objects or defects close to a new position on a wafer when the wafer is moved. This method is expected to give a high accuracy of correcting coordinates because objects or defects which are close to each other have a very similar coordinate error.
However, there is no means to check whether the coordinates are corrected to the expected frequency of occurrence of objects (assuming that all objects on the screen can be detected by the ADR). If the frequency of occurrence was low in an actual automatic measurement, the operator had to increase points of measurement for fine adjustment, increase the accuracy of correction of coordinates, and measure again. If an error of some tens micrometers is corrected down to some micrometers, objects and defects can be caught for observation. However, for accurate classification of objects and defects, images of a higher magnification are required. A typical manual detection of an object or defect after correction of coordinates by a fine adjustment comprises the steps of observing the specimen for an object or defect at a low magnification, moving the object or defect to the center of the screen, and increasing the magnification. An automatic object/defect detecting method comprises the following steps:
Measuring the locations of an object or defect on a wafer in advance by the other inspection system, moving the stage to the position of the object or defect according to the coordinate data obtained by the measurement, taking an image of the area including the object or defect at a preset low magnification (for searching), comparing this image by a normal pattern image which was obtained at the same magnification in advance, and thus detecting the object or defect. For investigation of objects or defects on a patterned wafer, the above detecting method further comprises the steps of taking an image of a pattern element (called xe2x80x9cdiexe2x80x9d) next to the pattern element including this object or defect at the same coordinates, comparing these images, moving the stage until the object or defect comes to the center of the screen, and shooting the object or defect at a preset high magnification.
In the above automatic detection, the setting of a searching is greatly restricted by a coordinate error. As the searching magnification is higher, the accuracy of detection of objects and defects can be higher. However, if the coordinate error is great, the objects and defects may not be in the screen field. Therefore, the operator had to move the stage to the coordinates corrected from measurement of some objects or defects after fine adjustment, set the searching magnification from the center of the stage and the actual position of the object or defect or the operator had to set a low searching magnification. In either case, the magnification to be set is dependent upon operator""s experience, skills, and so on and the final frequency of occurrence of objects or defects may also be affected. An object of the present invention is to provide a scanning electron microscope which can full-automatically search and classify objects and faults on a specimen without conventional technical faults.
To attain the above object, the scanning electron microscope of the present invention has a function for calculating the accuracy of correction of a coordinate correcting expression used for fine adjustment of linkage of coordinates when the scanning electron microscope automatically observes objects or defects on a specimen with electron beams and displays thereof for example by vectors; means for automatically detecting and storing information of coordinates of objects and defects fit to be used by a coordinate error calculating means; a function for automatically determining, from said information, a searching magnification which is used to automatically detect objects or defects after coordinate correction; and a function for calculating a frequency of occurrence of objects or defects and a required time from said searching magnification and conditions of measurement.
Further, the scanning electron microscope of the present invention has a function for displaying coordinate errors before correction by fine adjustment with vectors and a function for automatically determining, from said information, a searching magnification which is used to automatically detect objects or defects without a coordinate correction and calculating a frequency of occurrence of objects or defects and a required time. In other words, the scanning electron microscope of the present invention has a stage which moves with a specimen on it and a function for moving the stage to a corrected coordinate value which is obtained by correcting the coordinates of an object/defect of interest on said specimen which was obtained by the other inspection system. The scanning electron microscope further has a function for calculating the accuracy of correction of the coordinate correcting expression from said coordinate values and values of coordinates at which the object or defect of interest is observed.
The coordinate correcting expression can be made to minimize the difference between coordinate values of some objects of interest on said specimen which was by the other inspection system and coordinate values in the coordinate system of the stage. Further the coordinate correcting expression can be made to convert coordinates of some pre-selected points on a specimen into coordinates in the coordinate system of the stage for measurement in the coordinate system of the stage. The correction accuracy data is used in various ways. For example, the data is stored in a numeric table or displayed using vectors. It is preferable that the scanning electron microscope of the present invention has a function for visually showing the accuracy of correction on the position of a point of interest by displaying a vector equivalent to a difference between a position vector which represents coordinates at which the point of interest is actually observed and a position vector representing the corrected coordinates.
The other device can be an inspection system. The object to be observed is a foreign object or defect on the surface of a specimen. In the present invention, said object to be observed is a foreign object or defect. Judging from the accuracy of correction and the size of the observed object or defect, it is preferable that the scanning electron microscope has a function for automatically determining a searching magnification at which an object or defect on the stage at the corrected coordinates is automatically detected. Further, it is preferable that the scanning electron microscope of the present invention has a function for calculating the frequency of occurrence of objects or defects which are automatically detected at the determined magnification.
Further, it is preferable that the scanning electron microscope of the present invention has a function for calculating a time for measurement from the calculated frequency of occurrence, a condition of measurement, and the number of objects or defects. This condition of measurement contains auto-focusing condition, the number of frames which are added to an image to increase the S/N ratio, etc. The present invention can provide a scanning electron microscope which can properly correct a coordinate error between devices and automatically perform investigation jobs from defect searching to reviewing.